December's Sky Features Saturn At Its Best

Saturn is December's showpiece for observing. Nothing quite matches the
feeling of beholding it in a telescope for the first time -- yet it holds a
certain fascination even for experienced observers. Watch for it in the
east/northeast as it rises about 45 minutes after sunset. It's easy to
find: Just seek out the yellowish "star" that outshines the other stars,
except for Sirius and Canopus.

As the second-largest planet in the solar system, Saturn beats Earth in
both diameter (75,000 miles, or nine times) and mass (95 times). It also
trumps Earth with the speed of its rotation. In less than 11 Earth hours,
Saturn spins all the way around. This may be aided in part by the fact that
gaseous Saturn has a density less than that of water: Theoretically, if you
could find a bowl large enough, Saturn would float!

Instead, this beauty appears to float in space, buoyed by its rings. At
their fullest -- when the rings are tilted most toward our line of sight --
they reflect more light than the disk of the planet itself. In Saturn's 29
Earth-year-long journey around the sun, the rings slowly change their
orientation to us in a 14-year cycle. By 2009, Saturn's rings will orient
edge-on to Earth and nearly disappear. However, the present tilt is large
enough to reveal divisions in the rings. In September of 2001, Saturn
tilted its rings toward Earth at about 26 degrees, about the maximum possible.
They will remain at this angle for the rest of the year, providing an
excellent opportunity to study their structural nuances.

As you gaze upon Saturn through a telescope, you may notice a dark gap in
the rings. The largest of these dark splits, the Cassini Division,
separates rings A and B. It was first noticed in the 17th Century by
Giovanni Domenico Cassini. Another dark, thin line completely within the A
ring is known as the Encke Gap. The C ring -- also called the crepe ring --
is darker, and D darker still, and both are between the bright B ring and
Saturn itself. The remaining rings (labeled through G) are identified in
their order of discovery rather than placement. They are actually farther
out from Saturn than the A ring and cannot be seen from Earth.

Astronomers believe this dynamic ring system holds clues to the
evolutionary processes of the solar system. Ring elements range in size
from microscopic particles all the way to kilometer-sized chunks (composed
mostly of water ice). Pioneer 11 as well as the Voyager 1 and 2 probes
during the late 1970s and early 1980s revealed a variety of structural
features, such as sharp edges, arcs, waves, clumps, and kinked and braided
configurations. Occasionally, spokes may appear in the lighter B ring,
caused by very small particles (1 mm) that reflect light differently.

Observers should also watch for some of Saturn's many moons, which at last
count stood at 30. Titan, the only known moon with a dense atmosphere, is
half the size of Earth. Other large satellites include Rhea, Iapetus,
Dione, Tethys, and Enceladus. Smaller but still sizable are Hyperion,
Phoebe, Janus, Epimetheus, Mimas, and Prometheus; all are difficult to
observe from Earth. Another of Saturn's moons -- Pan -- is among those
embedded in the ring system.

Any telescope that magnifies at least 30x provides a good view of Saturn,
while views through instruments in the 4-inch to 8-inch range are often
exquisite. And don't forget to look for the shadow cast by the planet onto
its rings, which lends an awesome 3-D quality to the display. Only
observers with high-resolution telescopes will get to enjoy features on
Saturn's cloud decks. If you are among the lucky, you may also spot the
pale equatorial region, a dusky equatorial band, and the darker polar
regions. And if you experience excellent seeing conditions, you may even
get a glimpse at Saturn's belt system.

Equipment considerations aside, anyone who makes the effort to catch a view
of Saturn this month won't be disappointed. It's one of the most compelling
sites in the December sky.